The invention relates to a central guide means for the piston rod of reciprocating machines.
It is known in the art to provide the piston rod of a reciprocating piston machine with a central guide means in the form of a guide piston rigidly connected to the piston connecting rod and to which the main connecting rod of the crank mechanism is pivotally attached. Because of the cranking forces transmitted from the main connecting rod to the guide piston, neither tilting movements by the guide piston, even though they be minimal, nor the tendency of the guide piston to shift away from the central axis of the cylinder, can be avoided. Due to the rigid connection between the guide piston and the piston rod, these deflecting movements are transmitted in an undesirable fashion to the main piston, which in turn leads to undue wear. The tilting and transverse movements of the guide piston also have a detrimental effect on the sealing, because the sealing rings must follow the same movements. Therefore, adequate oil wiping action is possible only by means of costly dual rods and by placing the guide piston and the main piston at a considerable distance from each other. In the case of cylinders closed at the bottom, wherein the piston rod is guided through the cylinder bottom cover by means of a packing gland, deformations of the packing gland occur and adequate sealing is impaired to the point where it may lead to leakage to the ambient of the compressed gas.